The present invention relates to a frequency hopping method for a mobile radio system, especially for a WDCT mobile radio system.
Frequency hopping is used as is known in communication systems, especially in mobile radio systems to improve the transmission reliability. For this purpose the carrier frequency is changed in defined intervals, whereby in a mobile radio system the mobile stations are prompted by the base station to hop frequencies. Equally the sequence of the frequency hopping, that is to say the sequence of the carrier frequencies to be used after each other is notified to the mobile stations by the base station.
A known mobile radio standard, in which for example frequency hopping is used, is the so-called DECT (Digital European Cordless Telephone) mobile radio standard. This mobile radio standard was developed by ETSI (European Telecommunications Standard Institute), in order to have a leading European system also available for so-called CT (Cordless Telephone) applications, that is to say for cordless telephony. The DECT-mobile radio standard provides very good transmission quality but with limited range. The use of cordless DECT-systems is therefore preferred in domestic homes or in business premises.
In the case of the DECT-mobile radio standard ten different carrier frequencies with an interval of 1728 kHz are used for frequency hopping. The frequency spectrum in total ranges from 1880 to 1900 MHz. The carrier frequencies to be used for frequency hopping are notified to the mobile stations by the base station via a control channel. For this purpose a control message is transmitted by the base station to the mobile stations, which informs the mobile stations of the carrier frequencies available to the base station for frequency hopping. This message covers a bit field where for ten possible carrier frequencies a bit is optionally set in each case to ‘1’ or ‘0’ by the base station. If the bit corresponding to a defined carrier frequency is set to ‘1’, this means for the mobile station that the corresponding carrier frequency can be used for frequency hopping.
With the previously described principle frequency hopping is limited to a maximum of ten different carrier frequencies. The sequence of the carrier frequencies to be used for frequency hopping is pre-determined. If during a transmission for example a disturbance occurs due to a collision with interference frequencies, a new transmission using the same frequency hopping sequence is attempted which however often fails if the same disturbance (for example microwave frequencies) still continue.
Equally in further later standards, like for example Bluetooth or the SWAP Standard, static frequency hopping methods are used, with the aid of which collisions with interfering signals also cannot be reliably prevented and as a result the performance capability of the system is inevitably impaired.
Furthermore the DECT standard was essentially developed for the European telephone market. Since however the need for a reliable cordless telephone system also exists in the American market the so-called WDCT (Worldwide Digital Cordless Telephone) mobile radio standard was developed by the applicant based on the DECT standard. The WDCT standard uses a frequency band of between 2400 and 2483.5 MHZ which is compatible with the American FCC provisions for unlicensed operation.
As well as the frequency band being used according to the FCC provisions further conditions however are imposed on the mobile radio system being operated which amongst other things also concern the frequency hopping method used in each case. Thus the FCC provisions require that the frequency hopping method must encompass at least 75 different carrier frequencies since each frequency must not be used or maintained for longer than 0.4 sec within a period of 30 secs. Each frequency must on average be used the same number of times. Further it is required that the carrier frequencies being used in each case must be selected from a pseudo randomly arranged carrier frequency list.